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The Most Interesting Parasite in the World: Americans Don’t Always Have Brain Parasites, but When They Do, it’s Toxoplasma Gondii

According to the CDC, over 60 million Americans are infected with the single-celled brain parasite Toxoplasma gondii. In the short-term, this probably isn’t something to worry about: unless you are pregnant or immunocompromised*, there is almost no chance you would even realize you had been infected. But research over the past decade has suggested that the parasite may have subtle yet significant long term effects on human health and behavior.

The way that T. gondi carries out its life cycle is surprising devious for an organism that is smaller than a white blood cell. The parasites can only reproduce inside felines, but they can carry out their intermediate phase in a variety of species including birds and rodents. Rats are among the most common animal that play host to T.gondi’s intermediate phase. When rats consume parasites from the cats’ feces, they become infected with the T. gondi. However, for the parasite to complete its life cycle, it must find its way back into a cat. To accomplish this, the rat must be eaten. And clearly, it is in a rat’s best interest not to be eaten by a cat. So, to promote the continuation of its life cycle, T. gondi causes certain behavioral changes in the rat.

These changes are both bizarre and fascinating. Infected rats become significantly more active and less afraid of open spaces, meaning they can more easily be captured by hungry cats. But even more nefariously, infected rats even seem to have a suicidal preference for areas that contain cat odors. Thus, the parasite ensures that the rat will be eaten, and that it can complete its life cycle. The elegance of this single-celled organism is truly mind-boggling.

But T. gondi doesn’t limit itself to rats; it can infect a wide variety of species including humans. And while there are rarely obvious signs of illness in people, subtle behavioral differences may be present. Interestingly, men and women seem to be affected in nearly opposite ways. In women, T. gondi infection is associated with higher intelligence, extraversion, and conscientiousness, while in men the parasite is associated with lower intelligence, conscientiousness and novelty-seeking. However, both infected women and men seem to show higher levels of worry and neuroticism.

It is impossible to prove causation as it is unethical for researchers to purposefully infect humans with the parasite. These personality measures thus remain only associations; it is possible, for example, that women with certain personality traits may simply be more likely to contract the parasite. Yet, given the effects that T. gondi has on the brain functioning of rats, it seems unlikely that these associations are merely coincidental.

This has interesting implications not only for individuals but for whole cultures as well. Given that in certain geographical regions nearly 100% of the human population is infected with T. gondii, the parasite may have shaped the behavior of enormous groups of men and women, thus delicately shifting the paths of entire societies.

*Immunocompromised individuals and infants of women with Toxoplasma gondii are at risk for developing eye and brain damage.